1. Field of the Invention
The field of the invention is that of electron tubes and in particular those having longitudinal electron beams and having a grid, such as TOTs (Inductive Output Tubes).
2. Discussion of Background
An IOT has an electron gun which emits an electron beam, a resonant cavity through which the beam passes and a collector which receives the beam electrons as they leave the cavity.
The electron gun has a generally concave cathode in the form of a portion of a sphere, a control grid and an anode. The concave face of the cathode emits electrons when it is raised to a high temperature. The electrons pass through the control grid and are attracted by the anode and then enter the resonant cavity, forming a longitudinal beam.
The control grid serves to modulate the emission of electrons so as to group them into packets before they enter the resonant cavity.
The cathode is generally formed by a porous body impregnated with an emissive material. The porous body is made of tungsten and the emissive material of barium, calcium and strontium aluminates. It starts to emit electrons in the region of 900.degree. to 1100.degree. C.
The control grid is very close to the cathode. The gap between the grid and the cathode is about a few tenths of a millimetre. The emissive material has a tendency to evaporate and migrate in particular onto the control grid and onto the anode. The grid heats up, on the one hand, because of the proximity of the cathode and, on the other hand, because of the electrons which strike it. The emissive material which has migrated onto the grid causes spurious electron emission which disturbs the operation of the tube. Since the anode is relatively far from the cathode, it remains relatively cool, and the emissive material which covers it is not too much of a problem.
Solutions have been proposed for eliminating this spurious electron emission. One of them recommends bombarding the grid with electrons in order to heat it and cause the emissive material covering it to evaporate. The frequency of this heating may be daily, for example before each start-up. Cleaning the grid by heating is a severe constraint on an emitter of a television which may operate in a remote spot with remote control. Long term, this heating may also cause degradation in the operation of the electron gun.
Another approach taken for avoiding this spurious electron emission consists in decreasing as much as possible the temperature of the grid during operation of the tube. A known solution is to use a cathode working at temperatures lower than those of conventional cathodes, thereby lowering the temperature of the control grid. This solution does not give good results either. The spurious emission phenomenon is not eliminated, merely retarded.
Another drawback encountered is that the spurious emission from the control grid limits the size of the cathode and consequently the electron current produced.